The present invention relates to the purification and disinfection of raw water with chlorine dioxide and the subsequent removal of disinfection by-products. More particularly, the present invention relates to a process for the production of potable water by treating water infected with microbial life with gaseous chlorine dioxide, stripping the treated water, and then removing disinfection by-products by ion exchange.
For many years, chlorination was carried out for the disinfection of raw water. However, it was discovered some time ago that when surface water is chlorinated, trihalomethanes (THM's) are produced. It has been determined that various trihalomethanes, such as chloroform, are carcinogenic. It has further been determined that chlorinated drinking water, when ingested by laboratory animals, has also shown signs of carcinogenic effects.
As a result of such determinations, a survey of large municipal water systems was undertaken to determine how serious the trihalomethane problem was for systems using surface water as a feed for the municipal water plant. The resulting study entitled "National Organics Reconnaissance Survey for Halogenated Organics" by James M. Symons et al determined that many of the municipal water systems exceeded the 100 parts per billion (ppb) maximum trihalomethane level set by the EPA. Facilities testing in excess of the 100 ppb maximum THM level were required to switch to alternate disinfection systems such as chlorine dioxide disinfection, or in a some cases ozone disinfection.
Unfortunately, switching from a chlorination system to an ozone or chlorine dioxide disinfection system does not solve all the problems.
For example, in the switch from chlorination to chlorine dioxide disinfection it is possible to decrease the THM content in the disinfected water. However, in the disinfection process chlorine dioxide is reduced to chlorite ion by oxidation of the microorganisms or organic chemicals in the surface water. There may also be unreacted chlorite ions from the chlorine dioxide generation which normally converts only about 90% of the sodium chlorite to chlorine dioxide.
The presence of chlorite ion in human drinking water is of substantial concern because of a fear of adverse health effects on dialysis patients and glucose-6-phosphate dehydrogenase deficient individuals. Because of these health considerations, the EPA has advised that the total concentration of ClO.sub.2, ClO.sub.2.sup.- and ClO.sub.3.sup.- not exceed 1.0 ppm in finished drinking water.
Ozone disinfection is also beset with various deficiencies. Namely, the ozone is very expensive to generate, unstable and possibly carcinogenic. Because ozone is unstable and exhibits a loss of concentration very rapidly in water, it is necessary to add a stable disinfectant before distribution through the water system to prevent bacterial formation in the distribution piping.
Another method of decreasing the THM content in treated water is the addition of chloramines. Chloramines are formed by reacting chlorine with ammonia in water and are capable of reducing the formation of THM's, but they are not particularly potent as a disinfectant and are particularly detrimental to certain individuals with specific kidney problems.
The systems discussed above are the major processes for the disinfection of water approved by the Office of Drinking Water within the EPA. Chlorine dioxide disinfection is the typical disinfection method of choice when trihalomethanes in a chlorinated water system exceed the maximum contaminant level set by the EPA. Furthermore, it is expected that the EPA will lower the maximum acceptable THM level from 100 ppb to 35-50 ppb in the future. This will add pressure to municipalities to move away from Cl.sub.2 in their disinfection step.
Most existing chlorine dioxide processes do not, however, focus on the possible adverse effects that the presence of disinfectant by-products, e.g., chlorite or chlorate ions, in the drinking water has on human health. Furthermore, presently available processes for the generation of chlorine dioxide permit the reaction product, sodium chloride, to be added to the municipal water system along with unreacted chlorite ion, and HOCl if excess chlorine is added in the generator. Sodium chloride is undesirable in the case of people suffering from hypertension. Chlorite ion, either from unreacted sodium chlorite in the generator or from ClO.sub.2 which has been reduced in reaction with biological materials or organic compounds, is undesirable due to possible health effects in individuals susceptible to low levels of chlorite ions. Hypochlorous acid is undesirable because it chlorinates any organic compounds and because of the reports linking it to carcinogenicity in laboratory animals.
Accordingly, it is an object of the present invention to provide a novel process for the disinfection of water that prevents formation of the objectionable trihalomethanes.
Yet another object of the present invention is to provide a process for the disinfection of water which removes all or virtually all of the objectionable by-products of the disinfection process.
Still another object of the present invention is to provide a novel process which yields a water product which is essentially free of trihalomethanes as well as substantially all the objectionable disinfection by-products resulting from the disinfection process.
Another object is to provide a novel process for efficiently preparing chlorine dioxide for use in such water treatments.
These and other objects, as well as the scope, nature and utilization of the invention, will become apparent to those skilled in the art from the following description, drawing, and appended claims.